A subcatchment is a hydrologic unit that generates runoff and produces a hydrograph that can be routed downstream. It can have two parts, pervious and impervious areas. A pervious area is a surface where water infiltrates into the upper soil zone, but in an impervious area, there is no infiltration.
Regarding your project, you can divide a study area into different subcatchments. The outlet of a subcatchment can be a node of the drainage system or another subcatchment.

Parameter Name Description Default Value
NHYD Hydrograph number.
NAME Name of the component.
System Type Specifies the junction’s system type, whether it belongs in the overland, storm, or sanitary system. Blank
Outlet The NHYD of the outlet node.
COMMENT 1, 2, and 3 Comment for the command.
Tag Optional category or classification.
Area [ha] Area of subcatchment. 5
*Width [m] Width of the overland flow path (m or ft). 500
Slope [%] Average surface slope (%). 0.5
Curb Length Curb length (if needed for pollutant buildup functions). 0
Storm Index The storm index of the storm to use with the rain gauge group. 1
% Imperv Percent of impervious area (%). 25
N-Imper Manning’s n for the impervious area. 0.013
N-Perv Manning’s n for the pervious area. 0.1
Dstore-Imperv [mm] Depth of depression storage on the impervious area (mm or in). 1.5
Dstore-Per [mm] Depth of depression storage on the pervious area (mm or in). 5
%Zero-Imperv Percent of the impervious area with no depression storage (%). 25
Subarea Routing Choice of internal routing between pervious and impervious sub-area. OUTLET
Percent Routed Percent of runoff routed between sub-areas. 100
Infiltration Infiltration method. **HORTON
Groundwater Groundwater flow parameters. NO
Snow Pack Name of snow pack parameter set (for snow melt analysis only).
LID Controls LID controls
Land Uses Assignment of land uses to subcatchment.
Initial Buildup Initial pollutant buildup on subcatchment.

*There are different methods to calculate the length and width of a basin. A simple way is the Equivalent Rectangle Method to determine the shape of the watershed.
L= (C√A+ √(C^2-1.2544A))/1.12
B= (C√A- √(C^2-1.2544A))/1.12
Which A is the area of the watershed (km^2). C is the Gravelius coefficient. L is the equivalent rectangular length (km). B is the equivalent rectangular width (km).

**There are five techniques to model the infiltration into the unsaturated soil zone in the previous area.

  • Horton
  • ModifiedHorton
  • GreenAmpt
  • ModifiedGreenAmpt
  • Curve Number